1. Field of the Invention
The present invention relates to an electrophotographic photoconductor that contains at least one specific type of diazapentadiene derivative in a photoconductive layer, and an electrophotographic method and electrophotographic apparatus.
2. Description of the Related Art
In recent years, information processing system machines that use electrophotographic systems have remarkably advanced. In particular, laser printers and digital copying machines, which record information optically by converting the information to digital signals, have shown remarkable improvements in print quality and reliability. Furthermore, the above have been used in combination with high-speed technologies, and applied to laser printers or digital copying machines capable of full-color printing. Under such a background, as for a required function of the photoconductor, having both high image quality and high durability is considered to be crucial.
As for a photoconductor that is used in a laser printer, a digital copying machine or the like with such an electrophotographic system, in general, one that uses an organic photosensitive material (OPC) is widely applied in terms of costs, productivity, pollution free characteristics and other reasons. The layer structures of the OPC photoconductor can be broadly classified into the following two: a single layer type and a functionally separation-type laminated structure. A PVK-TNF charge-transfer complex-type photoconductor, which is the first OPC put into practical use, is a single layer type of the former.
Meanwhile, in 1968, Hayashi and Regensburger separately invented a PVK/a-Se laminated photoconductor. Later, by Melz et al. in 1977, and by Schlosser in 1978, a laminated photoconductor whose photoconductive layer is all made of organic materials, i.e. an organic pigment dispersion layer and an organic low-molecular dispersion polymer layer, was announced. The laminated photoconductor is also referred to as a functional separation-type laminated photoconductor because of the concept that the laminated photoconductor includes a charge generation layer (CGL), which absorbs light to generate an electric charge, and a charge transport layer (CTL), which injects and transfers the charge generated by the CGL to neutralize a surface charge.
However, when being repeatedly used, a film of the organic photoconductor becomes scraped more seriously than an inorganic one. As the film of the photoconductive layer becomes scraped even more seriously, the following tend to be accelerated: decreases in charge potential of the photoconductor, degradation of light sensitivity, scumming associated with surface scratches of the photoconductor or the like, decreases in image density, or image degradation. Therefore, the wear and abrasion resistance of the organic photoconductor has been cited as a major issue. Furthermore, in recent years, the photoconductor has become smaller in diameter as electrophotographic apparatuses have become faster, or as the devices have become smaller in size. Therefore, increasing the durability of the photoconductor is considered an even more important issue.
As for a method of realizing an improved wear and abrasion resistance of the photoconductor, the following methods have been widely known: a method of adding lubricity to a photoconductive layer, hardening a photoconductive layer, or containing a filler in a photoconductive layer; or a method of using a high-molecular charge transport material instead of a low-molecular charge transport material (CTM) molecular dispersion polymer layer. However, when the scraping of the photoconductive layer is curbed by the above methods, a new problem arises. That is, it has been known that the following problem arises: ozone, NOx and other oxidizing substances, which are generated due to repeated use or the surrounding environment, are adsorbed on to a surface of a photoconductive layer; depending on repeated use or the usage environment, the resistance of an outermost surface is reduced, resulting in image deletion (blurred images) and other problems. According to the conventional method, a blur-generating substance has been gradually scraped off together with the photoconductive layer, and the problem has been avoided to some extent.
However, as described above, in order to meet the recent need for higher resolution images and higher durability, the addition of a new method is increasingly sought. One method to mitigate the effects thereof is a method of mounting a heater on a photoconductor. However, the method has become a major obstacle in making the device smaller in size and reducing power consumption. Among the effective measures to be taken are additive agents such as antioxidants. However, a mere additive agent does not have photoconductivity. Therefore, when large amounts of the additive agent are added to the photoconductive layer, the following and other problems associated with electrophotographic characteristics arise: lower sensitivity and increases in residual potential.
As described above, an electrophotographic photoconductor that is less seriously scraped off due to the addition of a higher wear and abrasion resistance or due to a process design around the photoconductor cannot prevent effects on image quality, including the occurrence of blurred images, decreases in resolution and other side effects; having both high durability and high image quality has been considered difficult. The reason is that higher resistance is suitable for suppressing the occurrence of blurred images, while lower resistance is suitable for curbing a rise in residual potential. It is difficult to solve the problem as both are in a trade-off relationship.
Most of the electrophotographic photoconductors in practical use are of a functional separation-type, in which a charge generation layer and a charge transport layer are stacked on a conductive substrate. For a charge transport material contained in the charge transport layer, a hole transport material is used. The above is exclusively used for a negatively charged electrophotographic process.
Moreover, a highly reliable charging method in the electrophotographic process uses a corona discharge. Most of the copying machines and printers employ the above method. However, as is well known, compared with a positive polarity one, a corona discharge of negative polarity is unstable. Accordingly, a charging method that uses a scorotron is adopted, contributing to an increase in costs. The corona discharge of negative polarity entails generating larger amounts of ozone, a substance that causes chemical damage. When being used for a long period of time, the problem arises that the image quality decreases due to: oxidative degradation of a binder resin and a charge transfer material, which is attributable to ozone that occurs at a time when charging takes place; and accumulation of, on the surface of the photoconductor, an ionic compound that is generated at a time when charging takes place, such as a nitrogen oxide ion, sulfur oxide ion or ammonium ion, for example. Accordingly, in order to prevent ozone from being discharged into the outside, a copying machine and printer of a negative charging type often use an ozone filter, contributing to an increase in costs of the devices. Moreover, large amounts of ozone generated cause problems of environmental pollution.
To solve the above problems, the development of a positive charging-type electrophotographic photoconductor is underway. The positive charging type can reduce the amount of ozone, nitrogen oxide ions and the like to be generated. Furthermore, when a two-component developing agent, which is currently widely used, is used, the positive charging-type electrophotographic photoconductor is better in obtaining a stable image with less environmental variation. Even in that regard, the positive charging-type electrophotographic photoconductor is desirable.
However, the single layer type of the positive charging type and a reversely-layered photoconductor have the following drawback: The fluctuation of properties associated with surrounding environmental gases, such as exhaust gases from blue heaters or cars, is large because a charge generation material, which is extremely susceptible to oxidizing substances such as ozone and nitrogen oxide ion, is positioned near the surface.
Meanwhile, in the case of a high-speed copying process, rather than the positive charging type as described above, the negative charging type is preferably used. The reason is as follows: an organic material, which shows charge mobility so high that no problem is posed for the high-speed copying process or the like, is substantially limited to a hole transport material that only has hole-transfer characteristics at the moment; in an orderly-layered laminated-type electrophotographic photoconductor in which a charge transport layer made of a hole transport material is positioned on the surface side, the electrification characteristic thereof is therefore limited to a negatively charged one in terms of operating principles.
As described above, regarding charging polarity, if the electrophotographic photoconductor can be used in both positively-charged and negatively-charged polarities, then it is possible to further expand the application range of the photoconductor. The expanded application range is beneficial in reducing costs with a reduction in the number of types of photoconductor, as well as in increasing the speed, among other things.
Under such a situation, an electrophotographic photoconductor capable of bipolar charging is disclosed in Japanese Patent (JP-B) No. 2732697. However, a diphenoquinone derivative, which is an electron transport material used in the electrophotographic photoconductor, is slightly lower in charge mobility. As a result, the sensitivity characteristics of the photoconductor are not sufficient enough to increase the speed of a copying machine or printer and to reduce the size thereof. Another disadvantage is that the repeated use thereof would blur an image.
What is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2000-231204 is an aromatic compound having a dialkylamino group, which is used as an acid scavenger for photoconductor. The compound is beneficial for image quality after repeated use. However, the charge transport ability thereof is low, making it difficult to meet the need for higher sensitivity and higher speed. Accordingly, there is also a limit on the additive amount.
Moreover, the following is disclosed in [Itami et al., Konica Technical Report, Volume 13, Page 37, 2001]: A stilbene compound having a dialkylamino group, which is disclosed in JP-A No. 60-196768, JP-B No. 2,884,353 and the like, is also effective for images blurred by oxidation-resistance gas.
However, the above has a dialkylamino group, which is a substituent having a strong mesomeric effect (+M effect) in a resonance region of a triarylamine structure, which is a charge transport site. Therefore, an ionization potential value of the whole is abnormally small. Accordingly, the charge retention capacity of the photoconductive layer that is used alone as a hole transport material becomes remarkably worse from the beginning, or due to repeated use. Therefore, the fatal disadvantage is that the practical application thereof is extremely difficult. Even when being mixed with other charge transport materials for combination use, an ionization potential value of the stilbene compound is far smaller than the above. As a result, the stilbene compound ends up being a hole trap site for mobile charges, resulting in an electrophotographic photoconductor that is extremely low in sensitivity and has large residual potential.
What is proposed in JP-A No. 2004-258253 is a photoconductor that contains a stilbene compound and a specific diamine compound and has the improved environmental tolerance for repeated use, acid gas and the like without causing a decrease in sensitivity.
However, the above is still not sufficient enough to make possible high-speed printing, or to make the device smaller in size in accordance with a smaller diameter of the photoconductor.
The use of a diazapentadiene derivative as decoloring dye for a heat developing material is disclosed (JP-A No. 2007-212741). However, the document does not suggest that the diazapentadiene derivative is used for an electrophotographic photoconductor.
The object of the present invention is to provide an electrophotographic photoconductor that is high in durability even when being used repeatedly for a long period of time; suppresses the degradation of images associated with a decrease in image density or the occurrence of blurred images; and can obtain high-quality images in a stable manner. Another object is to provide an electrophotographic method, electrophotographic apparatus, and electrophotographic process cartridge that can obtain a photoconductor that supports bipolar charging; uses the above photoconductor and therefore is not required to replace the photoconductor; makes possible high-speed printing or makes a device smaller in size in accordance with a smaller diameter of the photoconductor; and can obtain high-quality images in a stable manner even at the time of repeated use.